Themes are backgrounds for Super Mario Maker and change the Ground blocks.
Themes consist of:
Land - The default theme. (Mario does not change here)
Ghost House - A spooky ghost-house theme. Ground blocks change into brownish-grayish mansion-like blocks.
Underground - A cave-themed theme with stalactites hanging off of the top. It looks like a space-themed therein Super Mario Bros. 3.
Airship - A ship-like theme where ground is replaced by wood blocks. If enough are placed, windows/cannons may be seen.
Castle - A castle-like theme where lava is the floor and ground blocks are replaced by bricks.
Underwater - An underwater-like theme where ground blocks are replaced by sea-bottom blocks. If they are stacked directly upwards and there is 2 or more stacked, the blocks transform into coral blocks. This is 5he only theme where Mario's move set changes. Mario will swim instead of walking on ground. If Y is held while swimming, Mario will proceed to swim faster.